According to a conventional reactive ion etching method, with the use of a simple halogenized gas as an etching gas electric discharge has been made for processing inorganic and organic substances by ions, radicals, etc. in plasma. However, due to its low etching speed or unsatisfactory etching uniformity, there has been recently developed a new reactive ion etching method in which additive gases are mixed with the halogenized gas as etching gases.
For example, in an Al-film dry etching method with carbon tetrachloride or boron trichloride utilized as a reactive gas, chlorine is added for speeding up the etching speed, or hydrogen is added for improving the selection ratio of Al etching speed/resist etching speed. Additive gases for improving the etching speed or etching selectivity have a high activity as in the case in hydrogen, chlorine and hydrogen fluoride. Due to their explosive or injurious property, such gases are very dangerous and therefor inconvenient in view of operation as etching gases to be used in the rective ion etching gas method.